Expose property (e.g. InitialPosition) that will set X and Y start in CropCommandContext from CropTool
To achieve this with the current API this you can get the CropAdorner control from the visual tree and set its CropRect property.
private void imageEditor_Loaded(object sender, RoutedEventArgs e)
{
ImageEditorRoutedCommands.ExecuteTool.Execute(new CropTool(), this.imageEditor);
var cropAdorner = this.imageEditor.FindChildByType<CropAdorner>();
cropAdorner.CropRect = new Rect(10, 10, 100, 100);
}